Identification of strategies for minimizing bone demineralization and The objective of this longitudinal trial is to determine whether there are significant changes in bone mineral content and calcium homeostasis during lactation and the post-weaning periods, and whether these changes are influenced by dietary calcium intake. The specific hypotheses to be tested are: 1) there is a decrease in bone mineral content in mothers who are lactating for 6 months as compared to postpartum controls; 2) the decrease in bone mineral content during lactation is less in women with a high calcium intake; 3) there is an increase in bone mineral content post-weaning as compared to postpartum controls; and 4) the increase in bone mineral content is greater in women with a high calcium intake. Two randomized calcium supplementation trials will conducted, one during the first 6 months of lactation and the other during 6 months post-weaning. A 2-by-2 factorial design will be used for each trial, where the two factors are lactation (yes/no) or post-weaning (yes/no) and calcium supplementation (1000 mg supplement/day versus placebo). Participants (n=45 per treatment combination) will include healthy postpartum women between 20 to 35 years of age whose habitual dietary calcium intake is 800 mg/day or less. Lactating or post-weaning mothers and postpartum controls will be matched for race and age since these factors are known to affect bone density. Changes in total body, radius and lumbar bone mineral content will be determined using dual energy x-ray absorptiometry (DEXA). Additional information on several other factors known to affect bone mineral content, such as body weight, smoking, activity levels, and ovarian function, also will be collected. Information from this study will help explain changes in calcium homeostasis and bone mineral content that occur during lactation and the post-weaning periods, and determine the effectiveness of supplemental calcium intake in preserving or increasing bone mass during these periods. This is the first randomized intervention trial designed to answer these questions in a large cohort of women.